For millions of years, a tiny icy fragment wandered through space—like a sealed bottle drifting across a cosmic ocean.
This summer, that “message in a bottle” arrived in our solar system.
Scientists named it 3I/ATLAS, only the third known interstellar comet, meaning it came from another star system entirely.
When researchers from Auburn University pointed NASA’s Neil Gehrels Swift Observatory toward it, they made a groundbreaking discovery: a faint glow from hydroxyl gas (OH), a chemical sign that water was escaping from the comet.
This marks the first time water-related activity has been detected in an interstellar comet.
Normally, telescopes on Earth can’t detect such faint ultraviolet light because our atmosphere blocks it. But the Swift space telescope orbits high above Earth, where it can observe wavelengths invisible from the ground.
This allowed scientists to spot what would otherwise be an impossible signal—proof that 3I/ATLAS is losing water vapor as it travels through our solar system.
Water is a key ingredient in understanding comets. In those that belong to our solar system, scientists use water as a baseline to measure how active a comet is and how sunlight makes its icy surface release other gases.
Detecting the same signal in an interstellar comet means astronomers can now start comparing these alien visitors directly with our own comets—offering clues about how planets and comets form in other star systems.
What surprised scientists most was where this water activity was happening. Swift spotted the hydroxyl gas when 3I/ATLAS was almost three times farther from the Sun than Earth, well beyond the distance where water ice can usually evaporate.
Despite the cold, the comet was still losing about 40 kilograms of water per second—the equivalent of a fire hose spraying at full blast.
This strong ultraviolet signal suggests that something unusual is happening. Scientists think sunlight may be warming small icy grains that have broken away from the comet’s surface.
These grains might then vaporize, creating the surrounding cloud of gas. Such behavior has been seen in only a few distant comets and hints that 3I/ATLAS has complex, layered ices that record the conditions of its home system.
Each interstellar comet discovered so far has told a different story. The first, ‘Oumuamua, appeared dry; the second, Borisov, was rich in carbon monoxide; and now ATLAS is releasing water far from the Sun. Together, they show just how diverse the chemistry of planetary systems can be—and how the ingredients for life’s chemistry might exist throughout the galaxy.
“Every interstellar comet is a surprise,” said Dr. Zexi Xing, the study’s lead author. “Each one challenges our ideas about how planets and comets form around stars.”
As 3I/ATLAS fades from view, astronomers plan to observe it again after mid-November, when it becomes visible once more.
For now, its faint ultraviolet glow offers a powerful reminder: somewhere beyond our solar system, other stars are shaping icy worlds with the same essential ingredient that supports life on Earth—water.
